I'm interested in buying a 135 & using my 330ci (2003) as trade-in. I bought my 330 at a local dealership and kind of tested the waters on pricing/trade-in value for the 135. Not sure I'm getting the best deal, but am hesitant to go out of the local area. Any advice on this, recommendations? Problems, benefits, the hidden stuff you don't find out about until it's too late?

It would seem the major issues are threefold, whether you buy out of area or not:
-- Delta between wholesale & retail: dealers need to purchase your trade-in at ~ wholesale so they can make a profit on its sale (directly or indirectly). Ergo, do you want to 'work' for the delta between wholesale and the price you think you can sell your car? You can give them the difference by accepting their offer, or you can earn it for yourself; your choice.
-- Cash Flow: can you financially handle the purchase independent of selling your car...and/or cover the interim between sale and 135i purchase with a rental? Or maybe a family member will loan you a car, or the time frame will be short, or...but bottom line, if your cash flow doesn't make an independent sale of your current car possible, you have limited options & the dealer is the benefactor.
-- Other Factors: Was your current car damaged significantly, making resale more difficult but a trade-in more inviting. Is your current workload heavy, intruding on your ability to show your car/handle calls, etc.? Are you (time for some self-truth...) not a very good negotiator or salesperson? Sometimes, trading in can be a wise choice...but only sometimes.

well..I know you can run into some trouble if you buy out-of-state. I was living in NY and I purchased my 128 in FL before I moved down there, and since I didn't have my Florida address yet, they wanted to charge me NY taxes and there were some other issues that came up because I didn't yet live in FL...but I just waited a few days until I had my FL address and then there were no problems.